1. Field of Invention
The present invention relates to a method of treating a semipermeable membrane, which leads to an improvement in the selective separation performance of the membrane. Also the durability of the membrane, as measured by its effective operating life, is extended.
2. Description of the Prior Art
Up to the present, there have been various kinds of semipermeable membranes proposed as selectively separable membranes employing reverse osmosis process and such semipermeable membranes have been widely applied such as to the desalination process of sea water and brackish water, or to the recovery process of valuable materials.
As for above described semipermeable membranes, there are various kinds of semipermeable membranes, one of which, for instance, is an asymmetric membrane, that is to say, so-called "Loeb type" membrane represented by cellulose acetate membranes and another type which is a composite membrane formed of a microporous substrate and an ultrathin film formed thereon as a barrier layer having semipermeability, being made of polymers which are different from those which constitute said substrate, such as polyamides, polyureas and cross-linked polymers based on furfuryl alcohol and so on.
Semipermeable composite membranes have such characteristics or merits over so-called "Loeb type" membranes as follows;
(1) Optimum combinations of substrate and ultrathin film or barrier layer can be designed and selected according to purpose and object for applications.
(2) There is no necessity for semipermeable membranes to be preserved or stored in water or in wet condition but preservation in dry condition can be adopted.
(3) In general, they are more durable to hydrolysis by acid or alkali or deterioration by microorganisms as observed especially in cellulose acetate membrane.
As the well-known semipermeable composite membranes, for instance, there are ones which are formed of a microporous substrate made of polysulfone, polyvinyl chloride or cellulose acetate and a thin film formed thereon, as a barrier layer, composed of cross-linked polymers based on furfuryl alcohol (U.S. Pat. No. 3,926,798) and ones prepared by a barrier layer composed of cross-linked polymers of polyethyleneimine (Laid-open Japanese Patent Application Publication No. 127,481/1977 (Tokkai-sho No. 52-127481)) and so forth
Inventors of the present invention as well had already proposed a semipermeable barrier layer of which is composed of cross-linked polymers obtained by the polycondensation with furfuryl alcohol and such polyfunctional alcohols as inositol or sorbitol, tris-(2-hydroxyethyl) isocyanurate, tris glicidyl isocyanurate in the presence of sulfuric acid or the like as catalyst, and it was found that said semipermeable composite membrane held the characteristics and merits of aforesaid composite membranes, and besides exhibited extremely selective separation performance not only for sea water or brackish water but also for a lot of water-soluble organic compounds and inorganic compounds (Laid-open Japanese Patent Application Publication No. 24,602/1982 (Tokkai-sho No. 57-24602)).
These semipermeable membranes have structures such that a thin skin layer or barrier layer which substantially determines the reverse osmosis performance is supported by a layer called microporous layer or substrate, respectively, wherein the thickness of the skin layer or barrier layer is said to be about 0.1.mu.m and it is generally considered that the thinner the thickness is, the more preferable it is for the semipermeable membrane because water permeability becomes higher, wherein however, its quality is apt to be unstable. For example, in order that reverse osmosis is to be used in actual industries, semipermeable membranes are to be formed into membrane modules, and further as they are practically operated under such higher pressure as scores of kg/cm.sup.2, selective separation performance of the semipermeable membranes is generally decreased not only by the physical factors such as pinholes on the surface of the semipermeable membranes, few mechanical failures and embossing of the semipermeable membranes but also by such chemical factors as hydrolysis observed in the case of cellulose acetate membranes.
There have also been proposed attempts to maintain or improve the desalination performance from another point of view. Mainly for cellulose acetate membranes as the object, there are plenty of reports, wherein, for example, a procedure to dose surface active agents into the feed-water in Japanese Patent Application Publication No. 28,650/1973 (Tokko-sho No. 48-28650), in Japanese Patent Application Publication No. 32,869/1977 (Tokko-sho 52-32869) a method to improve the selective separation performance of the deteriorated membranes with polyvinyl alcohol, and further, in Japanese Patent Application Publication No. 13,431/1978 (Tokko-sho No. 53-13431) a method of improving the selective separation performance with co-polymers of vinyl acetate and unsaturated carboxylic acid are also proposed, respectively. Inventors of the present invention had also proposed membrane treatment agents mainly composed of vinyl polymers having saponified ester groups modified by carboxylic acid (Laid-open Japanese Patent Application Publication No. 114,306/1980 (Tokkai-sho No. 55-114306)).
About mainly synthetic semipermeable membranes as object, for instance, improving methods for selective separation performances with polyvinylmethyl ether and with tannic acid are proposed in U.S. Pat. Nos. 3,373,056 and 3,853,755, respectively. Furthermore, in PB report No. 82-157,975 a method for improving selective separation performance by repairing the failed parts of semipermeable membranes with various kinds of synthesized colloids is studied.
Inventors of the present invention also proposed an improving method for selective separation performance of such semipermeable membranes as consisted of cross-linked polymers involving furfuryl alcohol as an essential monomer component with water-soluble amines (Laid-open Japanese Patent Application Publication No. 11,005/1983 (Tokkai-sho No. 58-11005)) before, and the present invention has been worked out as the result of further endeavors.